This application is based on Japanese Patent Application Nos. 2001-143570 filed May 14, 2001, 2001-143571 filed May 14, 2001, 2001-146560 filed May 16, 2001 and 2001-165283 filed May 31, 2001, the contents of which are incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to an optical waveguide and a method of manufacture thereof and more particularly to an optical waveguide for novel functional optical integrated circuits using an optical functional material KTaxNb1-xO3 as the optical waveguide and a method of manufacture thereof and also to a method of manufacturing a crystal film for use with optical communication devices.
2. Description of the Related Art
Intensive research and development efforts are being made from a cost performance point of view to develop optical integrated circuits that integrate on a single substrate optical devices that perform emission, detection, modulation, and multiplexing and demultiplexing of light. This integration technology is expected to reduce electric power, enhance performance and reduce cost of these optical devices.
Conventional optical integrated circuits currently in wide use have a fabrication in which a waveguide structure is formed on a semiconductor substrate using SiO2 and polymers to process an optical signal launched from outside. The waveguide structure refers to a structure comprising an undercladding layer, a waveguide layer formed on the undercladding layer and having a refractive index higher than that of the undercladding layer, and an overcladding layer covering the waveguide layer and having a refractive index smaller than that of the waveguide layer. To realize a function of optical signal processing, the conventional optical ICs change an optical properties of the waveguide material as represented by ordinary and extraordinary refractive indices by applying external fields, such as heat, electric fields, magnetic fields and sound, thereby achieving such functions as multiplexing/demultiplexing optical signals and adjusting a transfer time.
However, since the waveguide materials currently available are limited to SiO2, polymers, semiconductors and a small range of nonlinear crystals, the changing of the optical properties as realized by the method described above is greatly restricted by the characteristic of the waveguide material used, thus imposing limitations on the applicable optical signal processing.
Under these circumstances, the use of a novel waveguide material KTaxNb1-xO3 is being considered. The optical functional material KTaxNb1-xO3 exhibits an optical second-order nonlinear effect. An optical nonlinear constant of this material is 1,200-8,000 pm/V, significantly larger than 31 pm/V which is an optical nonlinear constant of LiNbO3 for example.
Further, since this optical nonlinear effect is attributed to the displacement of positions of constitutional elements by the application of an electric field, the presence or absence of the optical nonlinear effect can be controlled by the application of an electric field.
The material KTaxNb1-xO3 undergoes a ferroelectric phase transition at a composition-dependent Curie temperature of between xe2x88x92250xc2x0 C. and 400xc2x0 C. At this Curie temperature as a boundary the material""s property changes significantly. For example, its dielectric constant greatly changes from approximately 3,000 to about 20,000. It is possible to create a new optical integrated circuit taking advantage of the ferroelectric phase transition. The Curie temperature varies depending on the composition x of KTaxNb1-xO3, and adding Li to KTaxNb1-xO3 to produce KyLi1-yTaxNb1-xO3 makes it possible to adjust the temperature range.
The fabrication process of an optical waveguide requires steps of first forming a waveguide material film and then performing patterning and etching on the film using photolithography or the like.
The currently used waveguide materials, however, are limited to SiO2, polymers, semiconductors and a small range of nonlinear crystals. Hence, the modification of optical properties as realized by the aforementioned application of heat, electric fields, magnetic fields or sound is greatly restricted by the characteristics of the waveguide material used. The conventional optical ICs therefore have a problem that the applicable range of optical signal processing is very narrow.
Further, the method of manufacturing an optical waveguide using the KTaxNb1-xO3 optical functional material described above also requires the fabrication process, similar to the conventional one, of forming a film of the waveguide material and patterning the waveguide film by photolithography. Therefore, even in using the novel waveguide material KTaxNb1-xO3, the conventional technology has a problem that the waveguide fabrication process is complex.
Another problem is that, although the waveguide fabrication is essential in obtaining a desired performance, a technique to form waveguides in a KTN crystal has not yet been established. This is attributed to the fact that ions that increase the refractive index and still do not degrade the nonlinear characteristic after diffusion has not been found.
The chemical vapor deposition (CVD) method vaporizes a material containing constitutional components and causes a desired reaction in a gas phase or on a substrate. Forming a waveguide material film by using the CVD method requires a volatile compound containing the constitutional components. In KTN or KLTN, as to the compounds of Ta and Nb, halide and alkoxide have high volatility and can be used as the starting material when the CVD method is applied.
As to K and Li compounds, there is not much information available about the materials which provide sufficient vapor pressures. In the case of K in particular, no material has been known which is effective for use with the CVD method. The essential reason for this is that alkali metal elements such as K and Li tend to be ionized easily and cannot easily be kept in a molecular state necessary for vaporization.
The present invention has been accomplished to overcome these problems and provide an optical waveguide and a method of manufacture thereof, the optical waveguide being capable of having a variety of characteristics not achievable with conventional devices and of forming a waveguide easily.
Another object of this invention is to provide a diffused waveguide and a method of manufacture thereof, the diffused waveguide allowing a KTN crystal to be formed into a waveguide by diffusing Li, a technique not achievable with conventional devices.
To achieve these objective, the present invention provides an optical waveguide comprising: an undercladding layer; a waveguide layer formed on the undercladding layer and having a higher refractive index than that of the undercladding layer; and an overcladding layer covering the waveguide layer and having a lower refractive index than that of the waveguide layer; wherein the undercladding layer is a substrate and the waveguide layer is formed from an optical functional material KTaxNb1-xO3 (0 less than x less than 1).
Further, the substrate is one of a KTayNb1-yO3 (023 yxe2x89xa61, yxe2x89xa0x) substrate, a MgO substrate, a MgAl2O4 substrate and a NdGaO3 substrate.
Further, the undercladding layer comprises the substrate and one of SiO2, KTaxNb1-zO3 (0xe2x89xa6zxe2x89xa61, zxe2x89xa0x), MgO, MgAl2O4 and NdGaO3 deposited on the substrate.
Further, the overcladding layer is formed from one of KTauNb1-uO3 (0xe2x89xa6uxe2x89xa61, uxe2x89xa0x), MgO, MgAl2O4, NdGaO3 and polymer.
Further, an optical waveguide comprises: an undercladding layer; a waveguide layer formed on the undercladding layer and having a higher refractive index than that of the undercladding layer; and an overcladding layer covering the waveguide layer and having a lower refractive index than that of the waveguide layer; wherein the undercladding layer is a substrate and the waveguide layer is formed from an optical functional material K1xe2x88x92vLivTaxNb1-xO3 (0 less than x less than 1, 0 less than vxe2x89xa60.5).
This invention is characterized in that the optical waveguide is formed from an optical functional material KTaxNb1-xO3 whose optical properties represented by an electrooptical effect (EO effect), an acoustooptic effect (AO effect) and a figure of merit are remarkably large when compared with those of conventional waveguide materials.
The optical functional material KTaxNb1-xO3 is a paraelectric crystal material and has a cubic structure with a refractive index of 2.4 at temperature higher than ferroelectric transition. When an external field is applied in the crystal axis direction, the resulting positional displacement of the constitutional elements produces an optical second-order nonlinear effect. The optical nonlinearity constant of this optical functional material is 1,200-8,000 pm/V, significantly larger than, for example, 31 pm/V which is the optical nonlinearity constant of LiNbO3.
The optical nonlinear effect is the result of the positional displacement of constitutional elements caused by the application of an electric field. Hence, the presence or absence of the optical nonlinear effect can be controlled by the application of an electric field. The material KTaxNb1-xO3 undergoes a ferroelectric phase transition at a composition-dependent Curie temperature of between xe2x88x92250xc2x0 C. and 400xc2x0 C. At this Curie temperature as a boundary the material""s property changes sharply. For example, its dielectric constant greatly changes from approximately 3,000 to about 20,000. It is therefore possible to create a new optical integrated circuit taking advantage of optical characteristic changes caused by the ferroelectric phase transition.
The Curie temperature varies depending on the composition x of KTaxNb1-xO3, and adding Li to KTaxNb1-xO3 can adjust its Curie temperature range.
Further, this invention provides a method of manufacturing an optical waveguide, wherein the optical waveguide comprises an undercladding layer, a waveguide layer formed on the undercladding layer and having a higher refractive index than that of the undercladding layer, and an overcladding layer covering the waveguide layer and having a lower refractive index than that of the waveguide layer, the method comprising steps of: using the undercladding layer as a substrate and forming on the substrate a structure constituting a crystal growth nucleation position; and growing a thin film of an optical functional material KTaxNb1-xO3 (0 less than x less than 1) into a rectangular parallelepiped with the structure as a center to form the waveguide layer.
Further, this invention provides a method of manufacturing an optical waveguide, wherein the optical waveguide comprises an undercladding layer, a waveguide layer formed on the undercladding layer and having a higher refractive index than that of the undercladding layer, and an overcladding layer covering the waveguide layer and having a lower refractive index than that of the waveguide layer, the method comprising the steps of: using the undercladding layer as a substrate and forming on the substrate a structure constituting a crystal growth nucleation position; and growing a thin film of an optical functional material K1-yLiyTaxNb1-xO3 (0 less than x less than 1, 0 less than yxe2x89xa60.5) into a rectangular parallelepiped with the structure as a center to form the waveguide layer.
An ordinary waveguide fabrication process involves depositing a film of the material for a waveguide layer over a large area and then patterning the film into a desired configuration of the waveguide rectangular in cross section by photolithography. This invention takes advantage of the fact that the waveguide material is KTaxNb1-xO3 crystal and, instead of the ordinary process described above, forms the optical waveguide rectangular in cross section in a single film making step.
The optical waveguide fabrication method of this invention requires depositing a thin film of KTaxNb1-xO3 with optical characteristics sufficient for light propagation, i.e., satisfactory crystal quality that produces such characteristics, and then forming the film into a predetermined structure at a predetermined location according to a design of the optical integrated circuit. Such an optical quality can be realized by a crystal epitaxial growth method. In a field of semiconductor crystal growth technology, an epitaxial growth method available that grows thin films having a high degree of lattice mismatch between a substrate and a thin film, as in the case of GaN-on-sapphire and GaAs-on-Si, is a micro-channel epitaxy (for example, T. Nishinaga and H. J. Scheel, xe2x80x9cAdvances in Superconductivity VIII,xe2x80x9d ed. By H. Hayakawa and Y. Enomoto (Springer-Verlag, Tokyo, 1996) p. 33). This micro-channel epitaxy controls the thin film growth nucleation position by a groove formed on the upper surface of a seed layer on the substrate and improves the crystal quality of the thin film by the horizontal growth of the thin film from the nucleation position.
In this invention, since the nucleation position and the thin film growth direction can be controlled, when the crystal material has a strong crystal habit, it is possible to create a structure enclosed by the singular faces of the crystal material. The KTaxNb1-xO3 crystal material used in this invention has a cubic crystal structure and a strong crystal habit which is constructed by the {100} singular faces, so that a rectangular thin film enclosed by the {100} planes is likely to grow. In the process of growing a thin film, the growth nuclei on the substrate are generated starting from where the surface energy of the substrate is smallest. When there are holes or grooves on a planar substrate, the areas of the holes or grooves have side surfaces in addition to the bottom surfaces, increasing the number of contact surfaces with which the material supplied onto the substrate comes into contact. It is apparent also from the classical theory of crystal growth that an increase in the number of contact surfaces lowers the surface energy of the areas of the holes or grooves and thus the probability of crystal nuclei being generated in these areas becomes higher than in other planar areas.
Therefore, by forming in advance holes or grooves in that substrate contact surface where a rectangular waveguide is to be formed, the KTaxNb1-xO3 crystal material can be made to start growing a thin film at the holes or grooves as the growth nucleation points and fill these holes or grooves. If the growth of the KTaxNb1-xO3 crystal material is continued, a growth in the horizontal direction of the substrate, i.e., along the free surface, also starts, in addition to the growth in the vertical direction of the substrate at the holes or grooves. At this time, as to the growth in the horizontal direction of the substrate, the film being grown is limited in shape by the {100} singular faces of the KTaxNb1-xO3 crystal material. Thus, a film of KTaxNb1-xO3 having a rectangular parallelepiped structure enclosed by {100} planes can be produced.
Further, this invention provides a diffused waveguide formed by diffusing ions in a crystal and using as a waveguide core an area of the crystal diffused with the ions and having a higher refractive index than those of surrounding areas, wherein the crystal has a composition of KTa1-xNbxO3 and the ions are Li.
Further, this invention forms a waveguide core with a higher refractive index than those of the surrounding areas by diffusing Li ions in the crystal of a composition of KTa1-xNbxO3.
That is, this invention is characterized by Li ions being diffused in the KTN crystal to form a core with a high refractive index. Li ions can be thermally diffused by substituting a K site and the KLTN crystal having the composition of K1-yLiyTa1-xNbxO3 also has a performance equal to or higher than that of the KTN crystal. Therefore, there is no possibility of characteristic degradation due to ion diffusion. Further, the relative index difference obtained by adding Li through thermal diffusion is 2% or higher, which is sufficient for the fabrication of a waveguide. This means that Li is an appropriate ion for forming a waveguide. The melt containing LiNO3 used for diffusion has a low melting point of 261xc2x0 C., which means that a stable melt can be obtained easily. This melt has a high water solubility so that, after the diffusion processing, it can be easily washed away with water. Thus, it has no adverse effects on the subsequent thermal diffusion processing in a gas.
As described above, the manufacture of a diffused waveguide using Li ions has advantages that it can control the refractive index without degrading its characteristics and that the diffusion process using LiNO3 is simple and can perform diffusion at low temperatures.
Further, this invention provides a method of manufacturing a crystal film having a composition of KTa1-xNbxO3 (0 less than x less than 1), the method comprising steps of:
introducing, in the form of gas flows into a reaction system having a substrate, xcex2-diketone complex of K (R is an alkyl group with a carbon number of 1 to 7, Rxe2x80x2 is an alkyl group or CnF2n+1, and n is 1 to 3) expressed by a general formula (1) as a first initial material component, 
at least one of a gaseous Ta compound and a volatile Ta compound as a second initial material component, at least one of a gaseous Nb compound and a volatile Nb compound as a third initial material component, and an oxygen-containing gas used as an oxidizer, and reacting these components in a gas phase or on the substrate to form a crystal of KTa1-xNbxO3 on the substrate.
The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings.